(1) Field of the Invention
This invention relates in general to an apparatus for detecting photo-electrons, thermions, and exo-electrons characterized by low values of energy that are less than 10 electron-volts and in particular to such an apparatus capable of detection without primary ionization on the entrance of such low-speed electrons into the detection field.
(2) Description of the Prior Art
A typical apparatus for detecting photo-electrons, thormions, and exo-electrons, hereinafter referred together to as "low-speed electrons", in the prior art is disclosed in published Japanese patent application No. 52-7785. This apparatus consists of a bell-shaped cathode with an inter-electrode space opened to the atmosphere through an opening formed at the bottom of the cathode, an anode supported in the inter-electrode space of the cathode, and a pair of first and second grids that are situated in a vertically spaced-apart relationship above the cathode opening.
With this arrangement, the potential across the cathode and anode is normally maintained at around 3.4 kilovolts. The first grid and the second grid are connected to voltage sources of 100 volts and 80 volts, respectively.
The specimen to be measured by the apparatus is placed below the low grid. The low-speed electrons emitted from the surface of the specimen, upon entering into the inter-electrode space of the cathode through the two grids, triggers a gaseous discharge there causing the anode to generate pulses. Since the pulses thus generated from the anode are proportional to the low-speed electrons emitted from the specimen, counting means counts them as a function of the number of low-speed electrons.
Also, it is so designed that, when the interelectrode space assumes, owing to this gaseous discharge, such a state that defeats the capacity of the detector to detect, the first grid is connected to an additional source for a higher voltage that acts to extinguish the gaseous discharge. Further, the voltage of the second grid is reduced to a negative level which acts to neutralize the positive ions generated in the gaseous discharge. In this way, the detector restores its initial optimum operating condition. This process continues to repeat until the detection is completed.
However, the prior art apparatus of this type have been found to pose a serious problem, because of their designs that the inter-electrode space of the bell-shaped cathode is in free communication with the atmosphere. Since the counting rate or the number of pulses generated by the anode at gaseous discharge, as observed by counting means, varies with changes in ambient temperature and atmospheric pressure, the detection accuracy of the apparatus is affected by the conditions of the operating environment, in an extreme case, to altogether unreliable degrees.